The Fruity Oaty Bar is an Easter egg on the original Serenity release and a special feature on the Australian 2-disc set.
I can't remember how or if it is on the 2-disc Special Edition release.

To find the Fruity Oaty Bars Easter egg on the original Serenty release:
From the main menu highlight play, but don't select it. Press the left arrow. A background icon on the right will light up. Press Enter on that icon and you will be treated to a special behind the scenes featurette on the making of the Fruity Oaty Bars commercial that "triggered" River into her first fight scene. Enjoy it. You will be singing the song for weeks.

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